With the German patent specification DE 198 44 767 A1, a method attaching markers to a medical instrument that are detectable under MRI is already known. The orientation of the instrument within the MRI device can be determined with these points. However, the respective allocation of the measured markers to the instrument markers is impeded due to the similarity of the signal-emitting substance to the instrument material. The non-availability of an instrument fixation to the patient proves to be a further disadvantage. Such fixation could be achieved by use of trocars. FIGS. 2, 3, 4, and 5 show a device ensuring a minimally-invasive approach to the brain through a hole in the top of the skull. Such trocar is already known from patent specification DE 197 26 141 and prevents the risk of the so-called Brain Shift, which signifies the uncontrolled shifting of the brain inside the surrounding skull during an operation. This problem is not limited to the neuro field, but occurs whenever shifting tissue is punctured. The disadvantages of this kind of trocars are the following points:                The adjustment of a navigation system adapting the devices to MRI imaging to such a neuro trocar is difficult.        The neuro trocar is manufactured of titanium alloy, so that it is depicted as a homogenous formation with indistinct rim demarcation in the MR image. A three-dimensional orientation is difficult to assess. This, however, is highly essential, with the neuro trocar, unlike a stereotactic system, having no own reference point as it is fixed to the patient.        
The invention presented herein aims to solve these and other problems.